Hoping to prevent another school year from being upended by the pandemic, President Joe Biden visited a Washington middle school Friday to push his new COVID plan, accusing some Republican governors of being “cavalier” with the health of children.

Biden’s plan, announced a day earlier, would require vaccinations for up to 100 million Americans and seek to ramp up virus testing. With those measures in place, he said, schools should present little risk for transmission of the coronavirus.

“I want folks to know that we’re going to be OK,” Biden said during an appearance at Brookland Middle School, a short drive from the White House. “We know what it takes to keep our kids safe and our schools open.”

But as the surging COVID-19 delta variant casts uncertainty over the start of a new school year — in some cases prompting schools to shut down after a few days — it’s unclear whether Biden’s plan will go far enough to prevent mass disruption. Biden has little direct authority over most schools, which are generally governed at the local level, and his plan faces sharp resistance from Republicans.

Under his expanded vaccine mandate, all employers with more than 100 workers must require them to get shots or test for the virus weekly. A separate provision requires vaccines for workers in Head Start programs and at schools operated by the federal government, affecting about 300,000 workers.

The plan does not explicitly require vaccines for teachers in locally governed schools, but some education leaders believe the employer rule will effectively amount to a teacher vaccine requirement in many states.

That part of the plan is being enacted through a forthcoming rule from the Occupational Safety and Health Administration. And in states with OSHA plans, teachers will be among those required to get the vaccine or face testing, according to an interpretation by AASA, an association of school superintendents.

It’s expected to apply to 26 states, including several with Republican governors who opposed Biden’s plan, such as South Carolina, Tennessee and Arizona.

Biden did not address that aspect of his plan on Friday. Instead, he urged states to issue their own vaccine requirements for school workers.

“About 90% of school staff and teachers are vaccinated — we should have that at 100%,” Biden said. “I’m calling on all of the governors to require vaccination for all teachers and staff.”

Governors in a few states have already ordered teachers to get vaccinated, including in California, Oregon, New Jersey and New York. But most leave it up to school districts, and some Republican-led states have barred vaccine mandates.

Biden on Friday rebuked Republican governors who vowed to fight his new rules.

“I am so disappointed that, particularly, some Republican governors have been so cavalier with the health of these kids, so cavalier with the health of their communities,” he said. “We’re playing for real here. This isn’t a game.”

But most of his remarks struck a more conciliatory tone than a Thursday speech in which Biden vented his own frustrations with those who remain unvaccinated. He returned to a message of unity on Friday, insisting that “we’ve got to come together” to beat the virus.

Speaking to students at Brookland, Biden applauded those who had already been vaccinated. If all of them get shots, Biden promised to invite them to a special visit at the White House.

He also held up Washington, D.C., as a model. The city has hosted vaccine clinics at its public schools, and 65% of children age 12 to 17 have gotten at least their first shot, a rate that Biden said is among the highest in the nation.

In a plea to America’s families, Biden urged parents to get teenagers and other eligible children vaccinated as soon as possible. He argued that it’s no different than standard vaccinations for measles, mumps, rubella and other diseases.

Among the greatest threats to his plan, however, is the large population of children who are still too young to get the shot. Most elementary and middle schools have children below age 12 who have not been approved to receive the coronavirus vaccines.

Speaking to those concerns, Biden said he supports an “independent” scientific process to review the shots but he also promised to make them available to younger children as soon as it’s safe.

“I will do everything within my power to support the Food and Drug Administration in its efforts to do the science as safely and as quickly as possible,” he said.

The expanded vaccine mandate does not apply to students, but some districts have moved to require shots for students. The Los Angeles district this week became the first major district to enact a mandate for students ages 12 and up.

In addition to vaccines, Biden’s plan aims to ramp up virus testing in schools.